1. Field of the Invention
The present invention relates to relates to an image forming apparatus equipped with a laser scan unit that irradiates and scans a laser beam on a surface of a photoconductive drum to form a latent image.
2. Description of the Related Art
A laser device used for a laser scan unit of an image forming apparatus is provided with a laser chip for emitting a laser beam and a photodiode for receiving a part of the laser beam for the monitoring of the output power of the laser chip. The photodiode converts the received laser beam into an electric signal and feeds back the electric signal to a control unit of the image forming apparatus. The control unit receiving the fed back electric signal outputs a predetermined current control signal to a laser driving circuit based on the electric signal. In this manner, an automatic power control of the laser chip is performed by the control unit.
Output power characteristics of the laser chip depend on the temperature of the laser device, etc. Thus, in executing printing operation, laser power needs to be optimally controlled by performing the above-mentioned automatic power control according to a change in temperature of the laser device due to heat generation caused by light emission of the laser chip. In a conventional image forming apparatus, the automatic power control is performed at every scanning so that the laser power can be properly controlled even at the beginning of printing when an increase in temperature of the laser device is remarkable, thereby applying excessive loads on the control unit. Especially under the situation where an improvement in processing speed of the image forming apparatus is required, time necessary for one scanning needs to be shortened. As a result, the loads applied on the control unit have increased more and more.
During printing, the control unit has tasks such as processing of an image signal and convey and control of recording paper sheet. With recent demand for high resolution, the number of image signals to be processed has been drastically increased and convey and control with a higher degree of accuracy has been demanded. Thus, to achieve both of these tasks and the automatic power control without failure, it is necessary to use a CPU having appropriate throughput capacity as the control unit. The CPU having high throughput capacity is very expensive, which contributes to a substantial raise in manufacturing costs of the image forming apparatus. Thus, such CPU is not suitable for, in particular, the image forming apparatus for the consumer public requiring lower price.
Besides, Japanese Laid-Open Patent Publication No. 4-291377 discloses a laser beam printer capable of making fine adjustments to the output power of the laser device so as to prevent variation in resolution due to variation in sensitivity characteristics of a photoconductive drum or the like. Japanese Laid-Open Patent Publication No. 6-175450 discloses a copying machine capable of preventing variation in density due to a minute variation of currents or noise in the machine.